Diet plays a major role in the growth and health of humans and other animals. Nutrients in the diet perform three primary functions in the body. They provide the materials for building, repairing, and maintaining body tissues; they help regulate body processes; and they serve as fuel to provide energy. Nutrients are often classified into five main groups: (1) carbohydrates; (2) fats; (3) proteins; (4) minerals; and (5) vitamins. For humans, the Food and Nutrition Board of the U.S. National Academy of Sciences has established Minimum Dietary Allowances (MDA) and Recommended Dietary Allowances (RDA) for a large number of nutrients. Although not nutrients, other components of the diet are also important. For example, water is essential for life and dietary fiber is necessary for the efficient functioning of the digestive system.
Selenium is an element of atomic number 34. It is considered one of the essential trace minerals in the human diet. Selenium is believed to be essential for the proper development and functioning of the heart. Selenium is also a key component of the blood cell enzyme glutathione peroxidase that is, in turn, believed to be an anti-oxidant. Anti-oxidants are molecules that scavenge, or remove, free radicals. Free radicals are molecules with a lone pair of electrons that are highly reactive and are believed to play a role in cancer. The sale of selenium dietary supplements has dramatically increased over the past few years. Most of the selenium dietary supplements are derived from artificial sources such as yeast cultures.
An excess of selenium is believed to be as deleterious as an insufficiency. Excess selenium causes damage to various organs, especially the liver, and can cause death. It is believed that the damage from excess selenium results from selenium's interference with the sulfur-sulfur bonds in proteins. Although the MDA and RDA for selenium have not been established, the U.S. Food and Drug Administration (FDA) has expressed concern about the excessive use of selenium dietary supplements. The FDA has recommended that the maximum daily intake should not exceed 150 micrograms of organic selenium.
Selenium is present in the soil, but the level of selenium in the soil varies widely around the world. In most parts of the world, the level of selenium in the soil is about 0.01 to 0.05 parts per million by weight (ppm) or lower. However, in some parts of the world, the level of selenium is much greater. In some parts of the Western United States, including all or parts of the states of Montana, South Dakota, Wyoming, Utah, Nevada, California, Arizona, and New Mexico, the level of selenium in the soil is about 1 to 5 ppm or greater. This level is about 100 times the level of selenium in most parts of the world.
The level of selenium in the soil is important because it directly affects the level of selenium in crops grown in the soil, and, through the food chain, the level of selenium in animals who eat the crops. The uptake of selenium by plants depends on the type of plant and on the pH of the soil. Selenium in alkaline soils is in the form of selenates that are readily taken up by plants, but selenium in neutral or acidic soils is in the form of selenites that are unavailable to plants.
Cereal grains are grasses that are grown and harvested for food. Cereal grains include wheat, rice, corn, barley, sorghum, oats, rye, millet, and buckwheat. The terms "cereal" and "grain" are used interchangeably. The edible portion of cereal grains is the seed, also known as the kernel. The seed typically consists of an outer coat, a germ (or embryo) that contains most of the seed's protein, and an endosperm that contains a large quantity of starch that serves as the source of energy for the embryo. Some cereal grains are eaten directly by humans after cooking, but most are first milled and/or processed into flour, meal, starch, and other forms.
Cereal grains contain selenium when grown in alkaline soils containing selenium. The selenium in cereal grains concentrates in the germ because selenium in the organic state is normally associated with protein. For example, when wheat is grown in selenium-rich soil, the level of selenium in the seed as a whole is at least about 0.2 ppm. If the seed is milled and separated into components, the level of selenium in the germ is typically several times greater.
Legumes are flowering plants that are instrumental in nitrogen fixation. Nitrogen fixation is the process by which nitrogen is taken from the atmosphere and converted to organic compounds that can be used by plants. Legumes are often raised for food. With some legumes, such as peas, beans, soybeans, and peanuts, only the seed pods are eaten. With other legumes, such as alfalfa, clover, and vetch, livestock and humans eat other components of the plant, including the leaves, stems, and flowers. These components are known collectively as plant material. Legumes appear to be very effective at uptaking selenium from the soil. For example, vetch grown in an alkaline selenium-rich soil can contain up to about 200 ppm selenium and can cause death to livestock if eaten in quantity.
Cereal grains, legumes, and their various constituent factions have been used as foods for humans and animals since prehistoric times. The use of selenium as a diet supplement for humans and animals, although recent, is also well known. However, a demand exists for an economical supplement to add selenium, Vitamin A, Vitamin E, folic acid, and fiber to the human diet.